1. Field of the Invention
The present invention generally relates to a semiconductor device structure and the manufacturing process thereof, more particularly to the design and fabrication of a high driving current multi finger bipolar transistor device.
2. Description of Related Art
In radio frequency (RF) application, higher device cut off frequency (fT) is needed. Although higher fT can be achieved by advanced RFCMOS technology, it is still unable to fully satisfy RF application requirement. For example, it is difficult to get fT above 40 GHz. It is also very costly to develop advanced CMOS technology to get higher fT. The devices with compound semiconductor material can get very high fT, but the usage is restricted due to higher material cost, lower wafer size and poisonous nature of most of compound semiconductor material.
SiGe HBT is a good choice of high fT devices. Firstly, the emitter injection efficiency and current gain can be increased due to band gap difference of SiGe base to silicon emitter. Secondly, base resistance can be reduced and fT can be increased by heavily doped SiGe base. Lastly, SiGe module is compatible with silicon process. Now SiGe HBT is widely used in high fT device application.
As illustrated in FIG. 1, a multi-finger structure is used in existing SiGe HBT to increase current driving capability. To realize such a multi-finger structure, arrange the emitter (E) or the base (B) at the center, and symmetrically arrange bases or emitters at both sides of the central emitter or base, in a formation as C/BE/BE...BE/B/C, wherein, a base (B) is used by neighboring emitters, collectors (C) are formed at both ends of the structure, and the two collectors are connected to each other by a deep buried layer formed below the multi-finger structure, so that an overall multi-finger device structure is finally finished.
FIG. 2 illustrates a single device structure of an existing SiGe HBT multi-finger device, which includes collector 114, base 111, and emitter 110. The collector 114 is a medium doped or low doped n type epitaxial layer formed on top of the n type heavily doped buried layer 102. The collector 114 is connected to a metal electrode 107 through the n type heavily doped buried layer 102, the n type heavily doped collector pick up 104, and the contact 106, wherein, the n type heavily doped buried layer 102 is formed on top of the substrate 101; the n type heavily doped collector pick up 104 is in the active region; the contact 106 is formed through the interlayer dielectric 105. The n type heavily doped collector pick up 104 is formed by a high dose and high energy ion implantation. The area of the collector pick up 104 is large, so that the sidewall capacitance of the collector is also large. Both sides of the collector 114 are isolated by shallow trench oxide 103. Due to the large area of the buried layer, a deep trench 115 filled with polysilicon is added to the bottom of the shallow trench oxide 103 between devices to reduce the collector to substrate parasitic capacitance. The base 111 is a p type in situ doped SiGe epitaxial layer; the base 111 is connected to the electrode through a polysilicon layer 108 and a contact. A silicon oxide dielectric layer 113 is formed below the polysilicon layer 108. The emitter 110 is consisted of an n type heavily doped polysilicon layer formed on top of the base 111. Silicon oxide spacers 112 are formed at both sidewalls of the emitter 111. The contact area between the emitter 110 and the base 111 is determined by the window size of the silicon oxide dielectric layer 109. When opening the emitter window, selective ion implantation at the center of the collector which just enclose the emitter window can be performed to adjust breakdown voltage and cut off frequency of the SiGe HBT. To keep cut off frequency less affected, when multi-finger structure is adopted to increase the current driving capability, collectors should be arranged at the outmost sides of the whole device structure, as shown in FIG. 1.
This SiGe HBT multi-finger structure is reliable and widely used. But the main disadvantages are: a buried layer of large size is formed through the whole multi-finger device to connect the two collectors at the outmost sides, which leads to large junction capacitance, and impacts the device speed. In the meantime, due to a great number of emitters but only few collectors, the current is concentrated in the buried layer and in the collector electrode, the collector junction size needs to be increased to avoid too high current density, which further increases the junction capacitance. Thus a multi-finger device structure only adopts two collector electrodes at both ends instead of multiple collectors in order not to increase junction area and impact device speed.